rTMS over the cerebellum can increase corticospinal excitability through a spinal mechanism involving activation of peripheral nerve fibres.

OBJECTIVES: Single-pulse transcranial magnetic stimulation (TMS) over the cerebellum affects corticospinal excitability by a cerebellar and a peripheral mechanism. We have investigated whether any of the long-lasting effects of repetitive TMS (rTMS) over cerebellum can also be attributed to peripheral effects.
METHODS: Five hundred conditioning stimuli at 1 Hz were given over either the right cerebellum using a double-cone coil, or over the right posterior neck using a figure-8-coil. Corticospinal excitability was assessed by measuring the amplitude of motor evoked potentials (MEPs) evoked in the right and left hand and forearm muscles. Hoffman reflexes (H-reflex) were also obtained in the right flexor carpi radialis muscle.
RESULTS: rTMS over either the right cerebellum or the right posterior neck significantly facilitated MEPs in hand and forearm muscles in the right but not in the left arm (n=8) for up to 30 min after the end of the train. rTMS (1 Hz) of the right neck area increased the amplitude of the H-reflex (n=5).
CONCLUSIONS: Much of the persisting effects of rTMS over the cerebellum on corticospinal excitability appear to be mediated through stimulation of peripheral rather than central structures. Moreover, the results show that rTMS over peripheral areas can cause long-lasting changes in spinal reflexes.